(1) Field of the Invention
The present invention relates to solid-state imaging devices used for digital still cameras and the like.
(2) Description of the Related Art
With widespread use of digital cameras and mobile phones with a camera, the market of solid-state imaging devices has been growing. Thus demands for solid-state imaging devices are not limited to the realization of high sensitivity and an increase in the number of pixels. There is a strong demand for realizing a slim camera to be mounted on a slim apparatus, and thus such a solid-state imaging device is required to receive wide-angle incident light. This means that the demand is to receive both wide angle incident light and perpendicular incident light, even though the focus distance of a lens is short.
At present, in a CCD or a MOS image sensor that is widely used as a solid-state imaging device, semiconductor integrated circuits, each of which includes a light receiving element, are arranged in a two-dimensional array. Each light receiving element converts a light signal from an object into an electric signal. The sensitivity of a solid-state imaging device is defined by the magnitude of output currents from the light receiving elements in proportion to the amount of incident light. Therefore, for the reception of wide-angle incident light, there is a need to guide the light coming from the wide-angle direction to these light receiving elements.
There is also strong demand for realizing low cost solid-state imaging devices. This is because such devices are main components of digital still cameras that are in a fierce price competition. Therefore, such CCD and MOS image sensors are also being increasingly downsized. As a result of the downsizing of such image sensors and the increase in the number of pixels, the size of a light receiving area per pixel is being reduced. Therefore, a technique of condensing light to light receiving elements is of importance.
FIG. 1 shows a cross-sectional view of a conventional solid-state imaging device. A light 102 (the light shown by a solid line) that perpendicularly enters a microlens 101 proceeds to a color filter 108. The color filter 108 allows the color component, included in the incident light 102, which corresponds to the color filter 108 to pass through. The light that passed through the color filter 108 is converted into an electric signal by a light receiving element 106. Since a comparatively high light condensing efficiency can be obtained, the microlens 101 is used in almost all the solid-state imaging devices. Note that such a structure of a solid-state imaging device is also disclosed in, for example, Japanese Laid-open Patent Application No. 5-251673 publication.
As described above, a solid-state imaging device that receives a wide-angle incident light is required to guide the wide angle incident light to light receiving elements.
However, in the case of using a microlens 101, the light condensing efficiency decreases as the incidence angle of a signal light is increased. In other words, as shown in FIG. 1, the light condensing efficiency is high in the case of an incident light 102 perpendicular to the microlens 101, but the light condensing efficiency is low in the case of an oblique incident light 112 (the light shown by a 1-margin line). The reason is as follows. Since the oblique incident light 112 is partly blocked by a metal wire 103 in pixels or is widely deflected by the microlens 101, a part of the oblique light 112 does not reach the light receiving elements 106.
A solid-state imaging device is composed of pixels that are arranged in a two-dimensional array. Therefore, in the case of wide angle incident light, the angle of light incidence to the pixels in the peripheral part is greater than the angle of light incidence to the pixels in the center part. In other words, the oblique incident light 112 enters the pixels in the peripheral part. Therefore, the light condensing efficiency of the pixels in the peripheral part is lower than the light condensing efficiency of the pixels in the center part.
Further, even in the case of an incident light perpendicular to the microlens 101, as shown in FIG. 1, the incident light 122 (the light shown by a dotted line) to the boundaries of pixels cannot be condensed at a high efficiency. The reason is that a part of the light does not reach the light receiving elements 106 because it is blocked by the metal wire 103.
Also, almost all of the solid-state imaging devices have color filers 108 for red (R), for green (G) and for blue (B) that are used for separating incident light into color components of the light. This is the cause of increases in cost.